Heat issues

[No fries]

So the heat quit working but I was able to get it back on "temporarily"
The main Blower would not kick on. Thermostat calls for heat and i get flame, but the blower does not come on. If i cut the power to the furnace to reset it. Then turn the blower fan to on instead of auto, we have heat. Not sure what the sensor is called but I assume this one above the burners is bad.
We have two units and I kept an old one for parts, is this the sensor that is not calling for the fan to kick on when on auto?

 

[RatLabGuy]

That's probably the flame sensor.
After the ignitor kicks on and the gas valve opens, the ECU checks if there's a flame going on. If that thing doesn't detect a flame (it's just a heat sensor) and close the circuit the ECU cuts the valve off and tries again. It will cycle 3-5 times them quit, preventing it from getting to the stage that would kick on the main fan.

Switching stat to Fan bypasses all that and just runs the fan.

You can pull the sensor and do a resistance test on it.

The good news is they are like $8 on Amazon

 

[No fries]

It was the primary limit switch. I had pulled that switch circled, switched it out and it worked. According to the exploded diagram it was the primary limit switch.
Thats three things off my old unit that saved me $$ now!

 

[No fries]

That's probably the flame sensor.
After the ignitor kicks on and the gas valve opens, the ECU checks if there's a flame going on. If that thing doesn't detect a flame (it's just a heat sensor) and close the circuit the ECU cuts the valve off and tries again. It will cycle 3-5 times them quit, preventing it from getting to the stage that would kick on the main fan.

Switching stat to Fan bypasses all that and just runs the fan.

You can pull the sensor and do a resistance test on it.

The good news is they are like $8 on Amazon

It would fire and run with the fan at the thermostat on on. So the flame sensor knew we were getting fire. Heat ran as normal as long as the fan was on on and not auto.